In a two-cycle internal combustion engine, an intake air amount is increased by utilizing the pulsation of exhaust and hence the charging efficiency is increased, by which an output of the engine can be improved. Therefore, in a two-cycle internal combustion engine required to improve the output, an exhaust control valve for opening/closing an upper part of an exhaust port and exhaust control valve control means for controlling the exhaust control valve according to the rotational speed of the engine are provided so that the opening degree of the exhaust control valve is adjusted to obtain an exhaust pulsation effect in the range from low-speed region to high-speed region of the engine.
As a control unit for the exhaust control valve, there are available a unit in which an electric actuator for operating the exhaust control valve and a sensor for detecting the opening degree of the exhaust control valve are provided to control the actuator so that the valve opening degree detected by the sensor coincides with a desired opening degree, and a unit in which force relationships between tension of a spring for urging the exhaust control valve to the closing side and the exhaust pressure of the engine is utilized so that the opening degree of the exhaust control valve is subjected mechanically to automatic control according to the change in the rotational speed of the engine.
Generally speaking, when the opening degree of the exhaust control valve is increased and the upper part of the exhaust port is opened, a large exhaust pulsation effect is achieved in a high-speed rotation region, so that the charging efficiency can be enhanced, by which the output of the engine can be improved.
Also, when the opening degree of the exhaust control valve is decreased and the upper part of the exhaust port is closed, a large exhaust pulsation effect is achieved in a low-speed rotation region, so that the charging efficiency of the engine can be enhanced, by which the output of the engine in the low-speed region can be improved.
In a conventional two-cycle internal combustion engine using a carburetor as fuel supply means, when the intake air amount is changed by the exhaust pulsation effect, the amount of fuel is automatically controlled, and the air-fuel ratio of air-fuel mixture is kept in a predetermined range. Therefore, the carburetor need not be controlled according to the change in intake air amount.
On the other hand, in a two-cycle internal combustion engine using an electronic fuel injection control unit, the intake air amount is estimated from an opening degree of a throttle and rotational speed of the engine or is estimated from the pressure in an intake pipe and the rotational speed, and a fuel injection amount necessary for obtaining a mixture with a predetermined air-fuel ratio is arithmetically operated based on the estimated intake air amount. Therefore, measures cannot be taken against the change in intake air amount caused by the change in the opening degree of the exhaust control valve, so that the fuel injection amount cannot be controlled properly depending on the change in intake air amount caused by the control of the exhaust control valve.
When the intake air amount is increased by the change in the opening degree of the exhaust control valve, if the fuel injection amount is not changed, the air-fuel ratio changes to the lean side, and the mixture becomes lean, which causes the engine to seize.
Inversely, when the intake air amount is decreased, if the fuel injection time is not changed, the air-fuel ratio changes to the rich side, and the mixture becomes rich, which decreases the output of the engine, deteriorates components of exhaust, and increases fuel consumption.
Also, when the intake air amount is changed by the control of the exhaust control valve and hence the amount of fuel required by the internal combustion engine is changed, the optimal ignition timing of the engine changes naturally. When the fuel amount required by the engine is changed, if the ignition timing deviates from the timing adapted to the fuel amount, the operation of the engine becomes unstable and the output thereof is decreased. Therefore, when the exhaust control valve is controlled, the ignition timing must also be controlled.
Thereupon, the applicants proposed previously, in Japanese Patent Application Laid-Open Publication No. 2002-195073, a control unit for a two-cycle internal combustion engine in which, by controlling the fuel injection amount and the ignition timing according to the opening degree of the exhaust control valve, even when the intake air amount is changed by the change in opening degree of the exhaust control valve, the air-fuel ratio of the mixture and the ignition timing of the engine can be kept in a proper range.
In the already-proposed control unit, fuel injection amount arithmetical operation maps and ignition timing arithmetical operation maps are prepared beforehand for a plurality of openings of the exhaust control valve, and the fuel injection amount and ignition timing are determined by interpolation by using the fuel injection amount arithmetical operation map and the ignition timing arithmetical operation map selected according to the opening degree of the exhaust control valve. Thereby, the fuel injection amount and ignition timing are kept in a proper range with respect to the change in the opening degree of the exhaust control valve.
According to the already-proposed control unit, when the intake air amount is changed by the control of the exhaust control valve, the air-fuel ratio of the mixture can be kept in a proper range by changing the fuel injection amount. Therefore, phenomena that, when the intake air amount is decreased by the change in the opening degree of the exhaust control valve, the mixture becomes rich, which decreases the output and deteriorates the components of the exhaust gas and that, when the intake air amount is increased, the mixture becomes lean, which causes the engine to seize can be prevented.
However, in the already-proposed control unit, measures have not been taken against abnormality of operation of the exhaust control valve.
The abnormality of operation of the exhaust control valve occurs when trouble is caused in a control system of the exhaust control valve or when trouble is caused in the exhaust control valve itself. The trouble of the control system of the exhaust control valve includes, for example, a failure of a sensor for detecting the opening degree of the exhaust control valve, a failure of a power source for the sensor, breakage of a signal line for transmitting the output of the sensor to a control circuit, and a failure of an actuator for operating the exhaust control valve. Also, when the exhaust control valve becomes difficult to move due to carbon and the like adhering to a movable portion of the valve, the operation of the exhaust control valve itself becomes abnormal.
Even when means for controlling the fuel injection amount or for controlling the ignition timing according to the change in the opening degree of the exhaust control valve is provided as in the case of the already-proposed control unit, in the event when the operation of the exhaust control valve becomes abnormal, the arithmetically operated fuel injection amount and the intake air amount become unmatched, so that an anticipated object cannot be attained.
In particular, when the abnormality of operation of the exhaust control valve occurs, the control of the fuel injection amount is not carried out properly and the mixture becomes lean, so that the engine may seize and be damaged. If the mixture becomes lean and the engine seizes, the engine cannot be operated further. Therefore, the operation of the vehicle and the like driven by the internal combustion engine cannot be continued, and thus the vehicle and the like must inevitably be left at the site. In particular, when an outboard motor stops on the sea, or when a snowmobile becomes in a stopped state on a snow-covered mountain, the driver may meet with a disaster. Therefore, a state in which the operation of the engine cannot be continued must be prevented to the utmost.